In the manufacture of ophthalmic lenses, the use of optical quality glass has been supplanted in recent years by plastic materials. Although plastic materials offer the potential of easier fabrication and the elimination of at least some of the grinding and polishing steps required to form a glass lens, this potential has not been fully exploited by prior art techniques.
Indeed, the majority of plastic ophthalmic lenses are produced today by grinding and polishing to prescription. Another large segment of the lens market is filled by so-called "stock" lenses, i.e., lenses manufactured to final power but not edged. In the grinding method considerable time and labor is required to fashion the finished optical surface or surfaces. The stock lenses are generally formed either by grinding and polishing, or by casting with thermosetting materials, and either approach is a lengthy and labor intensive process. In addition, the casting process requires a rather long curing time, during which the molds that form the lens surfaces are tied up. Thus a lens casting manufacturing facility requires a large number of molds, and the molds comprise a large capital investment.
Plastic fabrication techniques such as injection molding or compression molding offer the advantage of far quicker component production per mold, but these processes have intrinsic drawbacks which present difficulties in the formation of high quality optical lenses. Generally speaking, in injection molding a heated, liquid thermoplastic material is injected under high pressure into a mold cavity that is held at a temperature substantially below the solidification temperature of the plastic. Solidification occurs unevenly as the material cools, and dimensional variations and shrinking from the mold surfaces creates poor lenses. Injection molding machines are generally adapted to produce batches of identical items at a fairly high rate, and changing of the molds to produce differing items is time consuming and expensive.
Compression molding has been used to fabricate plastic lenses, generally using a preformed blank which is compressed in a heated, softened state between two molds to form a finished lens. A clear disadvantage of this approach is that the preformed blank must undergo more than one heating and cooling step, thereby requiring a large energy input. Furthermore, finished lenses vary in the volume of material incorporated therein, due to the variations in prescriptions, and it is extremely difficult to provide preformed blanks that provide the required amount of plastic material. Thus control of the thickness of the finished lens is problematic in compression molding of lenses.
There are known in the prior art attempts to combine injection molding and compression molding techniques to elicit the best characteristics of both approaches. Generally speaking, these attempts have not provided sufficient productivity to justify the expensive and complex machines required to undertake the combined processes. It has also been difficult to eliminate the flash and sprues that accompany plastic molding, necessitating manual intervention and cleaning procedures. This drawback is contrary to the goal of automated production of finishes ophthalmic lenses.